1. Field of the Invention
The present invention relates to chemical mechanical polishing (CMP) apparatus for polishing a substrate such as a semiconductor wafer. More particularly, the present invention relates to the carrier head of a CMP apparatus that holds the substrate against a polishing pad of the apparatus.
2. Description of the Related Art
Integrated circuits are typically fabricated on a silicon wafer. To this end, conductive, semi-conductive and/or insulating layers are sequentially formed on the wafer. After one or more of the layers are formed, the layer(s) is/are etched to create circuitry features. The surface of the wafer thus becomes increasingly non-planar as the layers are sequentially formed and etched. This non-planar surface presents problems in subsequent processes used to fabricate the integrated circuit, such as in a photolithography process. Therefore, there is a need to periodically planarize the surface of the wafer.
Chemical mechanical polishing (CMP) is a process typically used for this purpose. The CMP process is well-suited for planarizing wafers of various sizes, i.e., even large-diameter wafers, because the CMP process produces excellent uniformity in planarizing relatively wide areas.
The CMP process makes use of mechanical friction and a chemical agent for finely polishing the surface of a wafer. In the mechanical aspect of such polishing, a wafer is placed on a rotating polishing pad while a predetermined load is applied thereto, whereby the wafer surface is polished by the friction created between the polishing pad and the wafer surface. In the chemical aspect of such polishing, the wafer surface is polished by a chemical polishing agent provided in a slurry that is introduced between the polishing pad and the wafer. The slurry may also contain abrasive particles that assist in the mechanical polishing of the wafer surface.
Typical CMP apparatus are disclosed in U.S. Pat. Nos. 5,423,716, 6,210,255, and 6,361,419. In these CMP apparatus, a wafer is held by a carrier head. Then the surface of the wafer to be polished (the process surface or polishing surface) is placed against the polishing pad by the carrier head. At this time, the carrier head exerts a controllable pressure at the rear surface of the wafer.
More specifically, the carrier head includes a flexible membrane that provides a mounting surface to which the wafer is adhered, and a retaining ring to prevent the wafer from leaving the carrier head. The carrier head also includes a pressure chamber, and air inlets leading into the chamber. The membrane is expanded by feeding air into the chamber via the inlets. Thus, the load on the wafer is controlled by the amount of air fed into the pressure chamber of the carrier head.
Frequently, however, it is necessary to exert pressure on the wafer that varies from region to region across the wafer. To this end, the pressure chamber in the carrier head may comprise a plurality of discrete pressure zones.
For example, U.S. Pat. Nos. 5,964,653 and 5,916,016 each disclose a carrier head comprising a membrane having a plurality of annular flaps that divide the pressure chamber into a plurality of pressure zones. The upper portions of the flaps are fixed to a membrane support and the bottom portion of the membrane is divided by the flaps into sections that correspond to various regions of the wafer. The sections of the membrane in each of the pressure zones are expanded when air is supplied into the pressure zones. However, each section of the membrane corresponding to a border between adjacent regions of the wafer, i.e., each section of the membrane constituted by a flap, is not expanded. Accordingly, the lower surface of this section of the membrane forms a concavity that prevents the CMP process from polishing the wafer with a high degree of uniformity.
U.S. Pat. No. 6,390,905 discloses a carrier head comprising a plurality of annular ribs whose feet abut the wafer to form a plurality of discrete plenums corresponding to various regions of the wafer. A first set of air supply lines communicate with the plenums so that the pressure exerted on the various regions of the wafer can be controlled. Alternatively, the ribs may be attached to or in contact with a membrane to which the wafer is adhered.
However, in the carrier head disclosed in U.S. Pat. No. 6,390,905, a good seal must be established between the feet of the ribs and the wafer or the membrane. To this end, a secondary set of air supply lines leads to the heads of the ribs. The air fed through these air lines assists in the pressing of the feet of the ribs against the wafer or the membrane. In one embodiment, the foot of the rib has a rounded cross section similar to that of a toroid or an ‘elephant's’ foot. The patent discloses that the air pressure in the plenums also acts on the ‘elephant's’ foot to assist the air fed through the secondary set of air supply lines in creating a seal. Even this may be insufficient, in which case a vacuum line is provided through the rib.